Page 130 - Build Your Own Transistor Radios a Hobbyists Guide to High-Performance and Low-Powered Radio Circuits

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FIGURE 8-7 Alternative tuner section for the eight-transistor radio.
In this circuit, instead of using a nonsimilar two-gang variable capacitor for the
tuned RF and oscillator circuit, a twin 270-pF variable capacitor is used. Twin
variable capacitors are used not only for superheterodyne radios covering the
broadcast AM band or medium-wave band, but they are also used for long- and

short-wave radio bands as well.
For an AM radio with a twin 270-pF capacitor with a series 330-pF capacitor C16,
we need about 140 ~H of inductance. Because the 42IF100 oscillator coil T1 has
nominally about 330 ~H, a 220-~H or 270-~H fixed inductor is paralleled with the
main primary winding of T1 to provide a net inductance of about 140 ~H. Also, a

fixed 15-pF capacitor C22 is paralleled with the primary winding of T1 or with L2 for
better tracking with the tuned RF stage. In addition, the emitter resistor R2 (for the
oscillator circuit Q1) is changed from 1 k
n

to 510 to ensure more than 300 mV peak to peak of the oscillation Signal (at Q1's
emitter). Note that C3 is connected to the low-side tap of Tl. The output of the
oscillator is connected to mixer transistor Q2's emitter terminal through C4.

As mentioned in Chapters 3 and 5, using a loop antenna allows for more flexibility
because loop antennas can be made with about 10 turns around a 6-inch rectangle,
square, or ellipse or bought readily at MCM Electronics or on the Web. Thus a loop
antenna is connected to a second 42IF100 coil T7; it is connected to the low-side
tap that is used as an RF transformer. The other section of the twin 270-pF variable